Well I started running "properly" i.e. frequently, focussed,logging progress, etc in January with a view to doing London. Part of the training schedule for this was a half mara within a few months - think Fleet was my first one. Realistically I guess 3 months but I had a reasonable base level of fitness from with gym work etc. It really depends how you feel in yourself, how much work you're able to put in, whether you enjoy it or not and whether you stay injury free. I don't think I'm a a "natural runner", I suffer no guilt if I miss a run, probably consume too much alcohol, bix & cake but I tend to have a bit of an S&M approach to it whereby I make myself the underdog so I work harder - sounds odd I know.